Field of the Invention
The present invention relates to an image forming apparatus forming an image on a sheet, and a sheet processing apparatus processing sheets.
Description of the Related Art
Hitherto, when an image forming apparatus is used to carry out an image forming process, at first, a sheet is fed to an image forming unit to have toner images formed on the sheet, and then the toner images are fixed to the sheet via a fixing unit. Next, the sheet having the toner images fixed thereto is discharged through a sheet discharge port to an outer side of the apparatus. When subjecting the sheet to a binding process and the like, the sheet onto which toner image has been formed is conveyed to a sheet processing apparatus, where the sheet is subjected to a binding process and the like, and then the sheet is discharged through the sheet discharge port, and supported on a sheet support tray provided on the sheet supporting device.
When the sheet is heated in the fixing unit, the sheet may be curled upward or downward significantly at both end portions along a sheet discharge direction, depending on conditions. If the sheet is supported on the sheet support tray in such a state with both end portions curled, both end portions of the sheet may obstruct the sheet discharge port. In that case, the supported sheet may be pushed by the subsequently discharged sheet, and the alignment of the sheets may be deteriorated or the sheet may fall off from the sheet support tray.
Therefore, a sheet supporting surface of the sheet support tray has been formed conventionally as a ridge shape, having a first support surface, and a second support surface provided downstream in a sheet discharge direction of the first support surface and having a somewhat shallower angle than the first support surface with respect to a horizontal direction, in order to accurately support the sheet having both end portions curled. According to this configuration of the sheet supporting surface of the sheet support tray, when the curled sheet is supported, the sheet will follow the shape of the sheet supporting surface and the curl on the sheet will be cancelled.
There are increasing demands of forming images on a sheet having a high stiffness called super-thick paper, having a basis weight of approximately 350 g/m2 to 400 g/m2, and supporting such sheet on the sheet supporting device. The sheet having such high stiffness may have its leading end, i.e., downstream end in the sheet discharge direction, curled significantly downward. In that case, if the position of the sheet support tray is positioned close to the sheet discharge port, the leading end of the sheet may be caught on the sheet support tray, and the sheet may not be discharged appropriately. Further, the sheet may collide against a sheet already supported on the sheet support tray, and may push the already supported sheet off the sheet support tray.
Japanese Unexamined Patent Application Publication No. 2010-111477 proposes forming a sheet support tray capable of being moved up and down, and when a sheet having a high stiffness is discharged, the position of the sheet support tray is placed lower than when a sheet having a low stiffness is discharged.
However, in the case of such sheet supporting device disclosed in Japanese Unexamined Patent Application Publication No. 2010-111477, the sheet supporting surface of the sheet support tray is flat, so that the curl of the sheet having both end portions curled will not be cancelled by having the sheet supported on the tray. In that case, both ends of the sheet may obstruct the sheet discharge port, as described earlier, and the supported sheet may be pushed by the subsequently discharged sheet, and the alignment of the sheets may be deteriorated or the sheet may fall off from the sheet support tray. If the sheet support tray is too low, the discharged sheet may fall off from a downstream side in the sheet discharge direction of the sheet support tray.